The present invention relates to an optical filter element, and more particularly, to an optical filter element for a wavelength division multiplexing optical coupler and a wavelength division multiplexing optical coupler that uses an optical filter element.
A passive optical network (PON) is an FTTX (fiber to the x, where x is home, curb, etc.) network architecture in which optical signals are used for communications between a single station and a plurality of subscribers. In the PON architecture, an upstream signal that is transmitted from each subscriber to the station differs in wavelength from a downstream signal that is transmitted from the station to each subscriber. Further, an analog signal (image signal) that differs in wavelength from an upstream signal and a downstream signal may be multiplexed over an upstream signal or a downstream signal. An upstream signal may use light having a wavelength of 1310 nm, and a downstream signal may use light having a wavelength of 1490 nm or 1550 nm.
An optical line terminal (OLT) or an optical network unit (ONU) is arranged at the station and at each subscriber. Each of the OLT and the ONU needs a wavelength division multiplexing optical coupler for demultiplexing or multiplexing optical signals of a plurality of wavelengths.
FIG. 1 shows a conventional wavelength division multiplexing optical coupler 200. The optical coupler 200 includes an optical filter 206 having a predetermined transmission wavelength range. The optical filter 206 multiplexes and demultiplexes two wavelengths λ1 and λ2. The optical coupler 200 receives input light beams, into which light beams having different wavelengths λ1 and λ2 are multiplexed, from an input optical fiber 201. A lens 204 converts the input light beams into collimated light beams. The optical filter 206 reflects, toward the lens 204, light beams having the wavelength λ1 included in the collimated light beams. The reflected light beams are focused on a first output optical fiber 202 by the lens 204. The optical filter 206 transmits light beams having the wavelength λ2. The transmitted light beams are focused on a second output optical fiber 203 by a lens 205. The input optical fiber 201 functions as a common port of the optical coupler 200. The output optical fiber 202 functions as a reflected light output port. The output optical fiber 203 functions as a transmitted light output port. The optical coupler 200 is also referred to as a three-port coupler.
Japanese Laid-Open Patent Publication No. 1-295210 describes a three-port coupler including a dual collimator, a single collimator, and a bandpass filter. The dual collimator includes two optical fibers and a collimation lens. The single collimator includes a single optical fiber and a collimation lens. The bandpass filter is arranged between the two collimators.
Japanese Laid-Open Patent Publication No. 63-33707 describes a three-port coupler including an optical waveguide and a bandpass filter. The optical waveguide includes a Y-shaped branching portion. The bandpass filter is arranged on the branched portion of the optical waveguide.
Japanese Laid-Open Patent Publication No. 2003-240960 describes a three-port coupler using a gradient index rod lens as a collimation lens. The gradient index rod lens has a flat end face and is easily assembled together with an optical filter or an optical fiber. This structure enables the coupler to be downsized
Examples of optical filters include bandpass filters and edge filters (long wave transmission or short wave transmission filters). A bandpass filter selectively transmits light beams in a predetermined wavelength range. The transmission wavelength range of an edge filter is defined by two edge wavelengths (shortest wavelength and longest wavelength of the transmission wavelength range).
Japanese Laid-Open Patent Publication No. 54-17044 and U.S. Pat. No. 4,474,424 describe couplers that multiplex and demultiplex signals of three or more wavelengths.
Isolation for crosstalk attenuation is important in conventional wavelength division multiplexing optical couplers. Isolation is an index indicates the amount of light of which wavelength is not included in a predetermined wavelength range and which mixes with light beams that are provided to a reflected light output port or transmitted light output port. A higher isolation represents a lower amount of light that is not included in the predetermined wavelength range, and a lower isolation represents a higher amount of light that is included in the predetermined wavelength range.
Conventional optical filters fail to avoid reflection residues. Reflection residues lower the isolation of the reflected light output port of the filter. The reflected light output port included in the conventional optical coupler has an isolation of about 12 dB. The reflected light output port has an isolation of about 18 dB even when the transmission ripple is optimized. It is desirable to improve the isolation of all ports of the wavelength division multiplexing optical coupler to, for example, 20 dB or more.